1. Field of Invention
A new and useful process is directed to forming a block copolymer. The process is more directed to the forming of a block copolymer of two different polyamides having utility, after further processing, for example, as a fiber. The copolymer comprises blocks of many monomeric repeating units of each of the different polyamides. By way of comparison, a copolymer can comprise random sequences of repeating units of each of the different polyamides. The latter can be referred to as a random copolymer. Furthermore, a block copolymer and a random copolymer formed from the same two different polyamides are known to possess different properties.
2. Description of the Prior Art
Block copolymers of polyamides and methods for preparing said copolymers are known, see U.S. Pat. No. 3,683,047, Honda et al, issued Aug. 8, 1972. One disclosed method for producing a block copolymer involves mixing two different polyamide polymers at a temperature above the amide-interchange temperature of the mixed polyamides until the block copolymer is formed. By way of comparison if the aforementioned mixing at said temperature is of further duration the resulting product is a random copolymer.
The molecular weight of each of the polyamides used to make a block copolymer via amide-interchange can be relatively high, for example, 50,000-100,000. It is believed that at above an amide-interchange temperature an exchange can occur between the two different polyamide molecules at any location where ##STR1## exists. Thus it is possible that with two different polyamides, each of 50,000 molecular weight, the amide-interchange occurs right in the middle of the two molecules. As such a copolymer will have formed with one block having a molecular weight of 25,000 and the other block having an equal molecular weight while the copolymer still has a molecular weight of 50,000. Equally, the amide-interchange could occur towards one end of a polyamide and thus a copolymer could result having one segment, of say, 49,800 molecular weight derived from one polyamide, attached to another segment, of say, 200 molecular weight derived from the other polyamide. Because of apparent lack of control of where the amide-interchange will occur other methods have been suggested.
Honda et al suggests using two different low molecular weight, i.e. 1000-4000 polyamides. The polyamides are different, in part, in that one is an amino-terminated polyamide whereas the other is a carboxylic-terminated polyamide. The other difference resides in that the balance of the polyamides are also different. The aforementioned different polyamides are copolymerized at a temperature where amide-interchange or transamination is nominal while the reaction of amino-terminated groups with carboxylic-terminated groups occurs almost completely. The resulting product is a block copolymer wherein the blocks have essentially a molecular weight of the starting polyamide, i.e. 1000-4000.
The aforementioned methods require separate preparation of each of the starting components followed by remelting and mixing to make a block copolymer. This is a disadvantage.